Go into T Halls and stand/line-up at the top left of the plywood on the ground. Aim your crosshair at the middle of the fourth window on the right set of windows above Sunroom. Take two to three steps forward and left-click throw the smoke (with the steps it should hit at the bottom of the window). This smoke allows you to smoke off B Main at the beginning of the round and take Checkers control if the CT's have an aggressive awper, throw a Molotov at B Main entrance at the beginning of the round, etc.